Impedance inverter



Apn'l 16, 1940.

w. VAN B. ROBERTS IMPEDANCE INVERTER Filed May 1'7, 1958 BY, i

- /wvw :ATTORNEY Fig-Zshows an impedance invertingdevice ar- Patented Apr, 16, 1940 UNITED STATES PATENT-OFFICE IMPEDANCE INVERTER Walter van B. Roberts, Princeton, N. J., assignor to Radio `Corporation of America, a corporation of Delaware ApplicationvMay 1'7, 1938, Serial No. 208,410

Y -V 3 Claims. The present invention relates to impedance inversion both with and without a change of sign. In what followsthe term impedance inversion isused to denote the production ofan impedance which -is proportional to the reciprocal of`a given impedance.

The object of the invention is to provide means for inverting any impedance, with or without change of sign, and coincidentally independently controlling the magnitude of the inverted impedance and the relative amounts of power flowing into the original impedance element and the inverse impedance derived therefrom.

Impedance inverters have been used in the past of which the well known quarter wave 'length transmission line is an example. When the output terminals of such a line are connected to an arbitrary impedance the impedanc'e measured at the input terminals of the line is found to be the inverse of the impedance connected to 'the output terminals; specifically, the input impedance is equal to the square of the characteristic impedance of the line divided by the output fventio'n provides a means for impedance inversion 'which' operates independently of frequency and which permits control of the magnitude'of the inverted impedance without physical alteration of circuit elements'and also provides for independent control ofth'e amount of power deliveredto the impedance which is being inverted,y

as comparedwith the amount of power delivered to the points vpresenting the inverse impedance.

In the accompanying Adrawing:

Fig. 1 shows an impedance inverting device,

rar'igedto'provide an impedance which is inverse to a given impedance,v I

Fig. 3 shows an impedance inverting device,

inserted in a lter for providing uniform gain.

Referring specifically to Fig. 1, reference numeral 'If` represents a screen grid amplifier tube having its plate and gridcircuits energized with 4 suitable direct operating voltages by way of choke 2 and impedance 3 respectively and having its screen circuit connected to a suitable positive potential 'as indicated by the plus sign adjacent the lead to screen-4. The source I2 of grid energizing' potential is preferably adjustable. The/cathode of tube I is connected to a line'f-S whose terminal 6 acts as one input terminal? for the impedance inverting circuit, while .its ter,` minal I acts as one output terminal for the device. ACondensers 8 are blocking condensers. The otheralternating current input terminalis denoted' bynumeral ,9 and the other alternating output terminal by-numeral I0. So far there has been described merelyv a conventional screen grid amplifier tubev with means for separating its alternating input and output from its direct current energizing circuits;

arranged to provide negative transconductance between its Jinput electrode* I5 and its output electrode I6, 'is connected with its alternating "current input voltage'derived from terminal I0," 'while its alternating 'output is connected'tol terminal 9.. Its platej and grid circuits are energized in similar fashion .to the plate and grid:

circuitsgof tube I.. .-'In order that a `negative transconductance 4may be obtained from screen grid, tube Il, its. screenlll is maintainedjat a direct'potential suliiciently higherlthan the direct potential of plate I6, so that due to secondary emission from platev I6 the kcurrent flow tol plate II` is, reversed in direction but has 'a magnitudewhichis less the more V negative the potential of 'input electrode I5. The potential of platefl is, therefore, adjusted to y.the value .which produces ,ja maximum'of reversed current, jsothat the alternating plate 'circuit impedance is infinite or. at least very high. The operation of an'ordinary screen grid tube as a negative transconductance' device is m'ore fully described in Il.V S. Patent 2,093,781 granted to W. v. B. Roberts on september'ztigsv. f

If thetransconductance ofy tube I is yrepre- "lsented by G12 and theabsolute magnitude of the transconductance of tube II is represented by G21, then a positive increment of potential `impressed upon input terminal 9 causes an incre.-

:ment 'of' currentv G12. `to ow into the plate-of `tube I, as indicated in direction by the arrow alongsidethe'lead thereto. Similarly, a positive increment of potential rat output terminal JI0 causes a current to flow from vplate I6 of tube II yout through the input circuit. connected be- V In accordance with. the invention, however, a second screen grid tube II,

I tween terminals E and 9 as indicated by the nitude of this current is ZG12G21.

arrow above terminal 9. Thus the apparent impedance between input terminals 6 and 9, that is the ratio of the voltage applied thereto to the current flowing therebetween in response to said voltage depends upon the load impedance connected between terminals It and 'l across which Voltage is developed by the plate current of tube l Assuming an impedance Z connected between terminals lil and y7, it will be seen that the potential developed at terminal Il), per kVolt applied to terminal 9, is equal to -ZCua As a result of this potential being applied to input electrode I5 of tube Il a current flows from terminal G up through the input circuit to terminal`9 and thence, in the opposite direction to the arrow adjacent terminal 9, into the plate i6. The mag- Hence the apparent input impedance between terminals and 9 is l ZGiaGzx which impedance is by definition inverse to'impedance Z.

Referring now to Fig. 2, the rectangle marked TWT represents any device having the properties of the combination of tubes shown in Fig. 1. Since the arrangement shown in Fig. 1 is such that voltage impressed upon the input terminals determines the current flow between the output terminals, and vice versa, the. arrangement as a whole will hereinafter be referred to as a twoway` transconductance device, or for the sake of brevity, TWT. As was explained in connection with Fig. 1,. the input impedance measured between terminals 6 and 9 of Fig. 2 is Thisv impedance is inverse to the output impedance Z and its magnitude depends upon the product of the two transconductances. G12 and G21. Hence since it is well known that the transconductance of each of the tubes of Fig. l may be varied by varying the bias potential of its grid, the magnitude of the inverse impedance obtained between terminals and 9'of Fig. 2 can be controlled by a mere variation of-electrical potentials, thus permitting various types off automatic control which would not be feasible in case it were necessary to vary the'physical dimensions of circuit components.

The application of the arrangement of Fig. 2 to a few representative problems will now be discussed. Let rus Suppose that the output irnpedance Z consists of a pure inductance L'.v Then the apparent input impedance at terminals G and 9 will be that of a condenser whose capacity is GizGzlL. The magnitude of this effective capacity between terminals 6' and 9, however, may be adjusted by varying the product G12G21. Similarly, if Z consists of a capacity C then the inverse impedance obtained between terminals 6 and 9 will be that of an inductance whose value is and is similarly controllable.

If the transconductances o-f both devices of Fig. l should have the same sign, however, a change of sign occursalong with impedance inversion. The transconductances of the two de Vices l and ll of Fig. 1 .can be given the same sign by changing the screen and plate supply voltages of either to the same values as employed in the other. Thus an inductance L be tween output terminals I and l!) becomes a positive reactance between terminals 6 and 9, butl of signs at all frequencies, can be obtained be-k tween a given capacity and the input terminals of a TWT terminated lby an inductance, if the TWT is composed of transconductances of similar sign. The same remarks apply to universal resonance with a given inductance element. This phenomenon of universal resonance has many i possible applications, including resonant voltage rise independent of frequency for untuned amplifiers, and the wipingout of undesired susceptance in parallel with resistance coupling of a wide band amplifier. This last result is of course accomplished by simply connecting the input terminals, such as 6 and 9 of Fig. 2,.across the undesired capacity, and connecting across the output terminals 'l and I0 an inductance equal to the undesired capacity divided by the product of the transconductances ofthe TWT.

vReturning now to what will be called the normal construction of the TWT (that is, with transconductanoes of opposite signs) it will be demonstrated that the TWT device may be used to match any two impedances which are inverse to each other. Fig. 3 shows an example of this application.

In Fig. 3 the TWT device is shown inserted in i a lter of the so-called constant 7c type. lThis type of nlter is composed of series elements Z1 and shunt elements Z2 whose product is independent of frequency. 1n the drawing the lter is shown as extendingindenitely both to right and left as the number of sections employed has no relation to the present invention. In accordance with the invention, and as indicated by the impedance values attached tothe various elements, the right hand portion of the filter is terminated at mid-shunt where it is connected to the output terminals 'l and l0 of the TWT. Hence the impedance to which terminals 'l and I0 are connected is equal to 'I'he inverted impedance measured between terminals 5 and 9 in the absence of the left hand portion of the filter is then But this last expression is exactly the impedance of the `filter when terminated at mid-series.

Therefore an exact impedance matching between the left hand filter portion and the TWT is obtainable by terminating the-left hand portion of the iilter at mid-series as indicated on the drawing. Since the TWT device operates as an impedance inverter in both directions, transmission may be eiiected in either direction through the system without reection losses at the TWT, the only diierence between the two directions of transmission being in the amount of power gain that occurs in traversing the TWT. An improvement is thus eiected over the prior art of terminating a lter with a simple resistance that is not a perfect termination at all frequencies in order to insert an amplifying device for producing gain.

'Ihe power gain occurring in passing through a TWT device is calculated as follows: voltage e impressed between terminals 6 and 9 of Fig. 2, for example, produces a current eGizGziZ between these terminals, but at the same time causes a current :2G12 to ow between terminals 1 and I0. The voltage drop across Z is therefore eGizZ and the power developed in impedance Z is e2G212Z. The ratio of this power to the input power 2G12G21Z iS may be adjusted by varying both G12 and G21 in such a way that their product is not altered.

y Hence, in ine, Fig. 3 shows how a variable gain may be inserted in a filter without introducing any mismatching or reflection losses.

It will be understood that while a particular form of construction has been discussed for a TWT device, many other forms may be used for producing the same results. For example, a single electronic device having two sets of electrodes each arranged to have transconductance with respect to the other set could be used. Furthermore, it is not necessary that either of the component transconductance devices have a negative transconductance inherently, as the eiect of a negative transconductance may be obtained by a suitable reversing transformer or its equivalent. I therefore do not consider my invention to be limited to the particular construction of the TWT device shown herewith, but only in accordance with the followingfclairns.

What I claim is:

1. Means for producing between a first pair ofv terminals an apparent alternating current impedance which is inverse to a.' given impedance connected between a second pair of terminals, comprising in combination a one way `positive transconductance device having input electrodes connected to one of said pairs of terminals and output electrodes connected to the other of said pairs of terminals, and a one way negative transconductance device having input electrodes connected to that pair of terminals` to which the output electrodes ofv said positive transconductance device are connected, and having output electrodes connected to that `pair of `terminals to which the input electrodes of said `positive transconductance device are connected.

2. The combination dened in claim 1 wherein means are included for controlling the transconductance of one of said devices whereby to control the magnitude of said apparent impedance.

3. The combination dened in -claim 1 wherein said given impedance consists of a wave lter having one kind of termination whereby said apparent impedance is proportional to vthe impedance of a hypothetical second and similar l wave lter having an alternative kind of termi- WALTER VAN B. ROBERTS. 

